It is known that certain substances which may be present in the body can function as indicators for various pathological conditions in the body. Such substances are hereafter called indicator substances. Examples of indicator substances are glucose, lactate, pyruvate, glycerol, glutamate, and glutamine, cytokines and heart specific enzymes. Pathological conditions that may be indicated or detected, or as well forecasted, include ischemia, hypoglycemia, hyperglycemia, sepsis, cell membrane damage or lipolysis, vasospasms, metabolic disorders and inflammatory disorders. By measuring indicator substances, pathological conditions may be detected before they lead to clinical signs. It may even be possible to detect processes or conditions that eventually may lead to a pathological condition. In many cases it would be advantageous to have the possibility to measure the concentration of indicator substances directly in a blood stream, or in tissue fluid. However, until now there have not existed any systems suitable for clinical use for continuous measurement of indicator substances. Systems known from the background art all have different drawbacks. Examples of common drawbacks in background art systems are that the measurement delay is extensive and that those systems have measured phenomena that are the result of a pathological condition, e.g. ischemia. This is clearly disadvantageous. With measurement delay is meant the time that passes from the moment that a sample is taken until the moment that a measurement value relating to this sample is obtained. Also, in background art systems measurement values can often only be obtained with relatively extended time periods between each measurement value, e.g. if sample fluid is collected in micro-vials. In another system, according to prior art, blood samples are drawn from a patient before being analysed with a blood gas analyser. In a further system, also according to prior art, a microdialysis probe provided with a semi-permeable membrane is inserted into a vein of a patient. A perfusion fluid (perfusate) is pumped into an inlet lumen before entering a microdialysis chamber on the inside of the membrane. The perfusate absorbs substances in the blood through the membrane and passes into an outlet lumen of the probe and then flows through a sensor where the substances are measured.
U.S. Pat. No. 5,078,135 describes a measuring system where a drug is administrated to a rat and where a microdialysis probe is placed in the vein of the rat. Mass spectrometry is used to batchwise analyse the dialysate for obtaining pharmacokinetic data.
US-A1-2004/0191848 describes a system for measuring the concentration of glucose in tissue fluid. The system uses a microdialysis probe which is fed with a perfusate already containing glucose. The concentration of glucose in the perfusate is controlled using self-adaptive control.
In view of the prior art, there is a need for a more reliable and accurate measuring system that can be used in monitoring the condition of a critical care patient.